The 18-year-old midfielder was tipped for a first-team role earlier in the season but an issue picked up in the summer delayed his arrival.
Former United striker Saha wants to see the youngster "lean on" experienced midfielders Sofyan Amrabat and Casemiro as he continues to develop.
“It was an incredible start for Mainoo," he told BettingOdds. "Especially considering he has just come back from an injury.
"He showed confidence, willingness to receive the ball and had great positioning which at a young age is very impressive. It’s a good start, and now he needs to find consistency in his game to reach the top.
"He will have to lean on the more experienced players like Casemiro and Amrabat, both of which can really help his development.
"We can see straight away he has so much quality and he will be an incredible asset for Manchester United and Erik ten Hag, because he needs players like Mainoo who are energetic and aggressive."
Recent reports suggest England are confident that Mainoo will pick playing for the country of his birth over Ghana, having represented the Three Lions at a number of different youth levels.
And Saha reckons that a fasttrack into Gareth Southgate's senior squad may also be on the cards ahead of Euro 2024.
"A good comparison is Warren Zaire-Emery from PSG, who is just 17 and has had a run of games and now looks likely to go to the Euros with France," he noted.
"If Mainoo brings consistency and keeps playing at a high level then he could definitely give Southgate something to think about."